Abstract
This paper is the first of a two-part study that investigates summer convective afternoon rainfall (CAR) activity over the East Asian continent through simulation and projection. We focus on the CAR activity over Taiwan in Part I. In Part II, the changes in CAR activity over South China and Luzon are examined. Using the WRF (Weather Research and Forecasting) model driven by two super-high resolution global climate models: HiRAM (High Resolution Atmospheric Model) and MRI (Meteorological Research Institute Atmospheric General Circulation Model), this study evaluates the performance of models (HiRAM, MRI, and WRF) used to simulate the summer CAR activity over Taiwan. The evaluations focus on the spatial-temporal variations in CAR activity during two time periods: the present-day (1979 - 2003, historical run) and the future (2075 - 2099, RCP8.5 scenario). For the present-day simulations, analyses indicate that both HiRAM and MRI cannot accurately simulate the temporal evolution of diurnal rainfall. This timing shift problem can be fixed after dynamical downscaling using WRF. The WRF dynamical downscaling approach also helps generate a more realistic CAR amount simulation over western Taiwan, where most residents live. In future projections all models predict that a significant decrease in CAR amount will occur over southwest Taiwan. This decrease in CAR amount is suggested due to the decrease in CAR frequency, not a change in the CAR rate. All models suggest that the cause of decrease in CAR frequency over southwest Taiwan is weaker local afternoon surface wind convergence and thermal instability to suppress CAR genesis.
Highlights
Taiwan is a mountainous subtropical island located in the East Asian summer monsoon region (Fig. 1a)
Based on the diurnal rainfall observational characteristics (Fig. 2a, to be discussed later), a convective afternoon rainfall (CAR) day is identified as when (1) the accumulated rainfall during 1200 - 2200 local Taiwan time (LT) is greater than 80% of the daily rainfall, (2) the accumulated rainfall during 0100 - 1100 LT is smaller than 10% of the daily rainfall, and (3) the days affected by other weather systems are excluded
By comparing the Weather Research and Forecasting (WRF)’s simulated diurnal rainfall with the observation (Fig. 2a), the results show that both WRF-HiRAM and WRFMRI accurately depict the diurnal rainfall evolution, featured with a maximum value at 17 LT and a minimum value at 02 LT
Summary
Taiwan is a mountainous subtropical island located in the East Asian summer monsoon region (Fig. 1a). During summer (June, July, and August, or hereafter JJA), the rainfall in Taiwan is modulated by various types of weather systems (Chen and Chen 2003) Among these weather systems, convective afternoon rainfall (hereafter, CAR) is the most frequently observed pattern (Wang and Chen 2008). Studies have noted that CAR generally occurs on a day associated with a stronger sea breeze circulation during the daytime (e.g., Lin et al 2012). Such a temporal sea breeze circulation evolution is found to be consistent with the local temperature change (Huang and Wang 2014). These observational studies focused only on the CAR change in the past or present day, rather than projections for future change
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